1. Field of the Invention
The present invention generally relates to methods and systems for producing relatively high-purity bisphenol A products. More particularly, in one embodiment of the invention, the methods and systems relate to making a bisphenol A product of at least 99 weight percent purity that is formed while decomposition of bisphenol A is inhibited. In another embodiment, the methods and systems relate to forming adduct solids that contain bisphenol A and have a length to width ratio of less than about 5:1. Either one of the embodiments may be used in conjunction with the other embodiment.
2. Description of the Related Art
Bisphenol A ("BPA") is an important raw material in the production of epoxy resins and polycarbonate resins. Bisphenol A may be produced by various techniques, but typically it is prepared in an acid-catalyzed, condensation reaction of two moles of phenol and one mole of acetone. Commonly-used catalysts include hydrochloric acid, a mixture of sulfuric acid and hydrochloric acid, and an acidic form of an ion-exchange resin. A secondary catalyst may be employed to shift the reaction toward the production of the p,p isomer and away from the production of the o,p isomer and other impurities In the following description, it is to be understood that the term "bisphenol A" or "BPA" refers to the p,p isomer and not the o,p isomer, as the o,p isomer is considered an impurity. A BPA product having a purity of less than about 99.5 percent is usually unsuitable for making polycarbonates.
It is well known to practitioners of the art that exposure of bisphenol A to a temperature approaching or exceeding its pure melting point (about 157.degree. C.) may cause partial decomposition of bisphenol A to form phenol and impurities such as isopropenyl phenol. Isopropenyl phenol is a highly reactive species that polymerizes to form color body precursors that may be oxidized to become color bodies. Color bodies are undesirable species that increase the yellowness index of polycarbonate resins. The yellowness index is a measure of the clarity of the resin. The clarity of the resin increases as the yellowness index decreases. Temperature-induced decomposition of bisphenol A intensifies when the molar concentration of phenol is less than that of bisphenol A, with the rate of decomposition increasing as the concentration of phenol decreases relative to the concentration of bisphenol A. Thus, it is advantageous to maintain the temperature below about 150.degree. C. in any process step where the numbers of moles of bisphenol A present is greater than the number of moles of phenol present. Decomposition of bisphenol A increases as the time that the bisphenol A is exposed to a temperature above its pure melt point increases. "Heat history" refers to the amount of time that the BPA-containing medium has been exposed to temperatures in excess of the pure BPA melting point while the number of moles of bisphenol A present is greater than the number of moles of phenol present. Those skilled in the art recognize that the suitability of a bisphenol A product as a raw material to make polycarbonates and other selected materials is inversely related to its heat history. A significant heat history may render the bisphenol A product totally unsuitable for making polycarbonates and selected other materials.
Additionally, exposure of bisphenol A to oxygen and/or acidic species will tend to catalyze decomposition of bisphenol A. Therefore, a goal of practitioners of the art is to minimize the entry of oxygen and acidic species into the process. Small amounts of acidic species and oxygen are inevitably present in the process stream. Practitioners of the art tend to encounter problems when employing a vacuum system in the purification and/or recovery process, since such a system may promote air seepage into process streams, thereby providing additional oxygen for the formation of color body impurities.
The alcohol color is commonly used as a measure of the tendency of bisphenol A product used for making epoxy resins to increase the color of the epoxy resins. As the alcohol color of a BPA product decreases, the tendency of the BPA product to increase the color of an epoxy resin decreases. BPA products having an alcohol color greater than about 20 may be unsuitable as a raw material for some epoxy processes. The caustic color is commonly used as a measure of the tendency of bisphenol A product used for making polycarbonate resins to increase the yellowness index of the resins. As the caustic color of a BPA product decreases, the tendency of the BPA product to increase the yellowness index of a resin decreases. BPA products having a caustic color greater than about 15 tend to be unsuitable for making polycarbonates with low yellowness indexes.
In the preparation of bisphenol A by the reaction of phenol and acetone, practitioners of the art typically perform an initial purification step (i.e., the first adduct crystallization step) in which an adduct solid (i.e., adduct crystal) is formed that has a substantially equal number of moles of bisphenol A and phenol.
Some methods relate to recovering a bisphenol A product directly from the BPA-phenol adduct crystal without further intermediate purification steps. Bisphenol A product is then typically recovered. Often, these methods involve a second adduct crystallization to produce an intermediate grade product from the mother liquor (i.e., from the liquid effluent in the first adduct crystallization step) that failed to solidify in the first adduct crystallization. The intermediate grade solids are then typically recycled into the feed stream of the first adduct crystallization zone to increase the bisphenol A concentration in the feed stream and to increase the amount of bisphenol A relative to impurities in the first adduct crystallization zone.
Some methods relate to melting the adduct crystal to form a melt, and then stripping phenol from the melt in a falling film still or wiped film evaporator.
The above-described methods typically operate under a vacuum at a pressure of about 30-50 torr and expose bisphenol A to a temperature of about 180.degree.-200.degree. C. Trace amounts of phenol are then removed by steam stripping at a temperature typically about 180.degree.-200.degree. C., leaving a bottoms product melt termed "crude bisphenol A." The crude bisphenol A is then further purified in a medium other than phenol, with the medium typically being an organic solvent. Typically, the crude bisphenol A is crystallized from the medium, and then the bisphenol A product is typically melted and subjected to a distillation procedure to remove residual solvent from the melt before a bisphenol A product is recovered.
Some methods relate to redissolving the adduct crystals in clean phenol and again extracting a bisphenol A adduct crystal in a second crystallization step. Phenol may then be removed using a falling film still or wiped film evaporator, and a steam stripper, at temperatures as described above. The remaining finished bisphenol A is then solidified in a prilling or flaking process. Such prilling and flaking processes are well known in the art.
A variety of techniques exist for the recovery of a sufficiently pure bisphenol A product for use in polycarbonates, however it is believed that all such processes used by practitioners of the art expose bisphenol A to a temperature above at least 160.degree. C.
Chang et al. (U.S. Pat. No. 4,533,764) appear to disclose a method directed to "removing the remaining small quantities of solvent to a parts per million level", the solvent being "occluded solvent" that is present in bisphenol A "produced from solvent crystallization." Chang et al. mention solvents including methanol, acetone, methyl formate, benzene, toluene, xylene, 2-propanol, chloroform, methylene chloride, ethylene dichloride, and trichloroethane, however phenol is not stated as a solvent applicable to the Chang et al. process.
Iimuro et al. (U.S. Pat. No. 4,931,146) appear to disclose a process for obtaining high-priority bisphenol A by removing phenol from an adduct of bisphenol A with phenol and removing continuously the residual phenol by steam stripping, wherein a multi-tubular packed column is used as stripping equipment. The method of Iimuro et al., however, appears to subject bisphenol A to high temperatures (160.degree.-200.degree. C.) during the removal of phenol.
Jakob et al. (U.S. Pat. No. 5,269,887) appear to disclose a method in which phenol is removed from a BPA-phenol adduct using solid phase drying (sublimation). The method of Jakob et al., however, employs a vacuum. This vacuum tends to promote air seepage into the process. A goal of practitioners of the art is to minimize oxygen exposure in the system to prevent the formation of color bodies.
A number of other patents appear to be directed at purifying bisphenol A, including U.S. Pat. No. 4,354,046, U.S. Pat. No. 3,673,262, U.S. Pat. No. 3,290,391, U.S. Pat. No. 3,219,549, U.S. Pat. No. 2,791, 616, U.S. Pat. No. 3,326,986, U.S. Pat. No. 3,535,389, and U.S. Pat. No. 5,475,152. It is believed that the solvent leaching techniques presented in many of these references are typically performed subsequent to a high temperature distillation step in which a crude bisphenol A product is obtained. Such leaching techniques alone are believed to be insufficient to produce a bisphenol A product of adequate purity for use in polycarbonate resins.
All of the above-mentioned patents are herein incorporated by reference.
Adduct solids of bisphenol A have a natural tendency to grow in a long, slender shape. Practitioners of the art typically produce bisphenol A solids with a length to width ratio of at least 5:1. In the preparation of adduct solids that contain bisphenol A, the formation of "short," "fat," robust solids with the lowest possible length to width ratio is preferred to allow the formation of a stable and porous cake during recovery of the solids. As the porosity of the solids cake increases, the cake wash efficiency is increased and the deliquoring properties of the cake are enhanced. Practitioners of the art aim to create a gentle environment for solidification in order to prevent breakage of the solids. In addition, a gentle environment avoids turbulence that may induce secondary nucleation. Secondary nucleation tends to result in the formation of "fines." "Fines" are relatively small (e.g., less than 20 micron average width), undesirable solids that promote the formation of a tight, compact cake with poor deliquoring and wash characteristics. Tight, compact cakes have a large surface area to volume ratio and tend to hold excessive amounts of liquor. Practitioners aim to create larger solids to inhibit compacting of the recovered solids cake. To achieve the formation of larger solids, practitioners of the art maintain a low stream velocity in their crystallizers to prevent turbulence and breakage of the formed solids. Additionally, some practitioners of the art remove acetone and water from the composition from which the BPA-phenol adduct solid is formed. Acetone and water are removed from the composition prior to its introduction into a solidification unit where the adduct solid is formed. An effect of the removal of acetone and water is a significant increase in the viscosity of the composition, which impedes the formation of turbulence in the solidification unit.